In order to support the OTDOA feature, all eNodeBs (E-UTRAN Node Bs as would be understood by the skilled person) transmit Positioning Reference Signals (PRS) on a pre-defined time-frequency pattern. Whenever needed, the user equipment (UE) can receive a request to report the Reference Signal Time Difference (RSTD) for a set of eNodeBs, which corresponds to the time difference between the reception of the PRS sent by the measured eNodeB and the reception of the PRS sent from a reference cell (typically the serving cell). OTDOA requires the UE to measure the RSTD of up to 16 eNodeBs (for 3GPP Release 9) in order to be able to output a reliable measure of OTDOA.
To ease the detection, a location server in the network transmits OTDOA assistance data to the UE as specified by the LTE Positioning Protocol (LPP). This assistance data permits the UE to obtain a priori information on the range of RSTD to be estimated (a search window is provided and the UE detects the PRS on this window). An Expected RSTD parameter is provided to the UE together with the RSTD Uncertainty which indicates the uncertainty in the Expected RSTD value. The UE may then determine a search window over which the effective RSTD will be estimated, which will be a window of twice the Uncertainty value centered on the Expected RSTD value (see FIG. 4).
The search window may cover a significant portion of time and is not restricted to a duration of less than the cyclic prefix (CP). The most commonly used method to estimate the RSTD is a time-domain correlation of incoming signals with the time domain signal sent by the measured eNodeB (that can be computed based on the known time-frequency PRS pattern) to obtain the power profile of the propagation channel in question. Then, a peak detection algorithm is executed to estimate the first peak location.
If the search window size is less than or equal to CP length, an equivalent frequency-domain implementation using FFT, point by point multiplication with pilot sequence, and iFFT can be used. This approach is used in “Qualcomm Incorporated, 3GPP contribution R4-100739, Updated OTDOA link-level results for synchronous case, San Francisco, USA: 3GPP TSG-RAN WG4.” where the following is disclosed “In the receiver, the PRS pilot tones corresponding to each base station (BS) are extracted and correlation is carried out in the frequency domain. By taking the IFFT of the correlation sequence the channel impulse response is obtained.” However, this is not valid for higher timing ranges, which may be present since it is desired to measure delays corresponding to non-serving eNodeBs. Among the non-serving eNodeBs, one or more may be sufficiently far from the serving eNodeB to produce an RSTD significantly higher than the cyclic prefix (the latter being dimensioned to absorb not more than the delay spread of propagation channel between UE and serving eNodeB). In such a case, the RSTD uncertainty provided by the location server is not guaranteed to be smaller than the cyclic prefix. Notice also that the maximum authorized value for the RSTD uncertainty (as per the LPP part of LTE standards) is much higher than the cyclic prefix. Therefore, there is no way to choose an FFT window ensuring that the delay between the start of the window and the start of the received reference signal of the eNodeB to measure is lower than the cyclic prefix, as would be required for an optimal frequency domain processing using known techniques.
Therefore, currently, the only method that always works is the time-domain correlation, which is much more complex than an implementation in the frequency domain.
Accordingly, it is desired to derive a frequency-domain implementation of OTDOA in LTE networks that provides a less complex and hence more resource efficient solution.